"If anybody comes here, the broken thread will give us proof," exclaimed the Scoutmaster. "I do not suggest that anyone will come, but if they do this is the best means of detecting their presence without giving them cause of suspicion." "But the cotton only stretches for a distance of about ten feet, sir," remarked Phillips. "If anyone descended to the ledge from a point farther along the cliffs, this arrangement would not give any warning." "I am afraid, Phillips, you didn't use your eyes sufficiently this afternoon. If you had you would have noticed that at all other places except this the cliffs overhang the ledge, and anyone being lowered would drop clear of the path leading to the caves. Hence it is reasonable to conclude that anybody knowing of the existence of the caves would naturally know the easiest means of gaining the ledge. Do you follow?" "Yes, sir," answered the Second. "Now let us be off, back to the camp. Those fellows are enjoying themselves to be sure. You can hear them quite plainly at this distance--it is another sign that stormy weather is near when sound travels clearly over a long distance." Ten minutes after "Lights out" the camp at Seal Island was wrapped in slumber. Dead tired with their exertions and aided by the health-giving fresh air, the Scouts were soon lost to the world, till a blaze of red in the eastern sky betokened the dawn of another day. Before réveillé, Atherton was up and about. His mind was full of the tracks that had been discovered in the Tea Caves. It was not presentiment that influenced his thoughts. His deductions were based upon actual facts that were certainly suspicious. On the other hand the mysterious visitor might have gone to the cave for a perfectly legitimate purpose. In that case the following up of the clues would result in nothing more or less than a little practice. Something more than curiosity prompted him to run across the Island to the edge of the cliffs by the caves. Phillips had told him of the harmless and effective booby-trap that the Scoutmaster had prepared. The cotton had been snapped. Atherton knelt down and examined the ground, but the dew lay thick upon the long grass, and no sign of human footprints was visible. Upon his return to the camp, the Scout found Mr Trematon clad in a long overcoat, for the morning air was chilly. "The cotton has been broken, sir," announced Atherton. "I am not surprised," replied the Scoutmaster. "As a matter of fact I expected that it would be, for by a stupid blunder on my part I tied the thread without making allowance for its shrinking through the moisture of the night air. But apart from that, Atherton, I have made a discovery. Two of the thole-pins have been taken out of one of the boats." "Perhaps Mayne and Baker brought them ashore when they returned from the mainland last night, sir?"